Thus EP-A-618 243 states that a combination of a nonionic compound and an acetylene diol with triple bond results in improved viscosity. As appears from the examples, an approximate addition of 15-22% by weight of the nonionic compound and 1-5% by weight of the acetylene compound is necessary for a concentrate containing 18.5-25% by weight of an associative polymer and having a viscosity below 10,000 mPa.s to be obtained.
From EP-A-682 094 it is known to use as viscosity-reducing agent for associative polymers nonionic surfactants selected from the group consisting of ethoxylated alkyl phenols, ethoxylated alcohols and polyglycols. Even if the viscosity is essentially reduced, 20% of a nonionic surfactant is, according to Example 8, required to obtain a 25% concentrate of the associative polymer with a viscosity of 4,300 mPa.s.
EO-A-682 094 also states that anionic surfactants can be used, but they often cause undesired frothing and undesired effects in certain applications. For example, when present in colour compositions, they result in a dried colour coat with reduced water resistance.
EP-A-460 896 discloses the use of cyclodextrin to reduce the viscosity of an aqueous associative thickening agent.
From WO97/02325 it is known to use as viscosity-reducing agent for concentrates of thickening agents adducts having the formula EQU R.sup.1 O.paren open-st.R.sup.2 O.paren close-st..sub.n H
wherein R.sup.1 is an aliphatic hydrocarbon group having 6-10 carbon atoms, R.sup.2 is an alkylene group and n is a number between 3 and 7, whereas DE-A1-19 600 467 discloses, among other things, The use of a C.sub.8 alkyl glycoside as viscosity-reducing agent. Even if the nonionic surfactants in the two last-mentioned references often have a good viscosity-reducing capacity, they have a limited water solubility and/or a tendency to frothing in aqueous compositions. The surfactants may also cause trouble such as turbidity in coating compositions and their dried layers.
An object of the present invention is to be able to prepare an aqueous concentrate of associative thickening agents having low percentages of viscosity-regulating additives, which to a small degree affect subsequent applications. They should not cause turbidity in the coating compositions and their dried layers. A further desideratum is that the additives should not cause frothing and that they should be easily biologically degradable and nontoxic.
It has now been found that these objects can be achieved by using as viscosity-reducing agent for associative thickening polymers a nonionic surfactant having a hydrophilic group consisting of ethyleneoxy groups as well as propylyeneoxy groups.